1. Field of Invention
The present invention relates to a method and system for measuring an inner diameter of a hole in an object.
2. Description of Related Art
There has been known a non-contact measuring system for measuring the diameter of a hole in an object without contacting the object, wherein an image of the hole is enlarged by such means as a microscope. The distance between the ends of the enlarged image is measured using a measuring machine or an ocular micrometer. Alternatively, the image of the hole is applied to a charge-coupled device (CCD) camera so as to measure the dimension thereof.
These systems can only measure the diameter of the hole at a position near the surface of the object, and not inside the hole at an arbitrary position.
The applicant of the present invention has proposed in Japanese Laid-Open Patent Application No. 7-260439, a system wherein the diameter can be measured at any arbitrary position. In the system, the inner diameter of the hole is optically measured using the trigonometrical measurement method. Since an optical element must be placed inside the hole, the object to be measured by the system is limited to one having a large diameter. Hence, the system cannot be applied to a hole having a small diameter.
Japanese Laid-Open Patent Applications Nos. 7-27507 and 7-4919 disclose systems where the diameter of a hole having a relatively small diameter can be measured at an arbitrary inner position of the hole. Each of the systems has a laterally movable base, on which is mounted an annular object. On one longitudinal side of the base is placed a projection device for projecting a cross on an inner wall of the hole. On the opposite side of the base, a monitoring device for monitoring the projected image of the cross through an optical system is provided.
In order to measure the diameter in the hole in the object at an arbitrary position, the projection device is operated to form the cross on one side of the inner wall of the hole while monitoring the cross through the monitoring device. Thereafter, the base is moved so that the cross is projected on the opposite wall of the hole. The moved distance of the base is measured, and the inner diameter of the hole is calculated based on the measured distance.
In the systems disclosed in the above mentioned documents, the cross is projected on opposite positions of the inner wall. Namely, the positions of the two extreme sides of the hole are detected one at a time. In order to detect the positions, the base must be moved. Thus, it takes time to measure the diameter. Moreover, the position of the base must be precisely adjusted to accurately project the cross on the opposite walls. Hence, more time is necessary, and in addition, a measuring error is apt to occur.